When multiphase fluids flow in a conduit such as a pipe, the distribution of the phases is generally irregular or non-uniform in the conduit, especially where the conduit is deviated from vertical. Often one phase is flowing at a faster rate than the others. This is particularly the case where there is a gas phase and a liquid phase or when there is a continuous liquid phase with an immiscible liquid phase of different density dispersed therein. Consequently, it is desirable to know the volume fraction of each phase in the flow and the distribution of the phases in the conduit.
Various approaches have been proposed to measure volume fraction and phase distribution in multiphase flows. It is generally considered preferable that the measurement technique be non-invasive, i.e., that any sensors should be placed at the periphery of the conduit rather than being positioned in the flow itself. In cases such as flows from hydrocarbon wells, in which there is a conductive phase (water or brine) and a nonconductive phase (oil and/or gas), it has been proposed to use capacitive measurements to analyze the flow. U.S. Pat. No. 5,017,879 describes an arrangement in which electrodes are arranged around a pipe to measure the capacitance of the fluid as it flows past the electrodes. U.S. Pat. No. 5,291,791 describes a development of the technique described in U.S. Pat. No. 5,017,879 in which a series of electrodes are arranged around the pipe and are connected to a switching arrangement which controls tile function of each electrode. By controlling the switching arrangement so as to create a measurement configuration similar to that in U.S. Pat. No. 5,017,879, and continuously changing the switching arrangement, the configuration effectively rotates around the pipe. The measurements taken for each position of the configuration can then be integrated over a given number of rotations to average out variations in sensitivity of the basic configuration due to the distribution of the phases in the pipe. U.S. Pat. No. 4,074,184 proposes a somewhat different approach, again using a series of electrodes around the pipe and a switching arrangement. In this case, each electrode in turn is excited and the capacitance is measured at each of the remaining electrodes. The measurements are then integrated over a given number of "rotations" to determine the volume fraction of the phases.
Capacitive techniques using a series of electrodes around a pipe have also been proposed for tomographic flow imaging techniques in order to identify the distribution of phases within the pipe. Examples of these can be found in U.S. Pat. No. 5,130,661 and GB 2,223,850.
Insertion devices using microwave propagation have been proposed for measuring volume fractions in multiphase flows, for example in U.S. Pat. No. 5,101,163, U.S. Pat. No. 4,996,490 and GB 2,2262,807. However, these techniques are not applicable to non-invasive devices. An imaging system for active microwave tomography is proposed in "Cylindrical Geometry: A Further Step in Active Microwave Tomography", IEEE Transactions on Microwave Theory and Theory and Techniques, Vol. 39, No. 5, May 1991. In this system, a cylindrical arrangement of microwave antennae is described, the object to be imaged being positioned inside this arrangement. Each antenna in turn transmits microwave energy which is detected at the remaining antennas. An image of the object is reconstructed from the detected signals. There is no teaching in this document which relates to dynamic measurements such as those in flowing fluids.
The present invention seeks to provide a method and apparatus which can be used to measure multiphase flows such as those encountered from hydrocarbon producing wells.